This invention relates to a process for preparing a product containing ammonium orthophosphate and that is substantially free of ammonium polyphosphate. The product is prepared from ammonia and phosphoric acid, in which phosphoric acid and gaseous ammonia are contacted together by means of a two-phase spraying device provided with two coaxial ducts. The ammonia flows through the outer duct and phosphoric acid through the inner one.
A process of this type is known from British Pat. Specification No. 1,252,651. In the process described in the British patent the ammonia and the phosphoric acid are passed through a joint discharge nozzle of a spraying device and sprayed onto a bed of ammonium salt granules that are kept moving in a rotating drum. Inside the spraying device, the reactants come into only very imperfect contact with each other only while they flow through the nozzle. Consequently, the mixing is very incomplete and, as a result, the reaction is effected substantially in the bed of the solid particles in the drum. Using this process a homogeneous product is not obtained within the spraying device. To obtain a homogeneous product, spraying onto the moving bed of granules is necessary, so that homogenization can be effected in this bed. Another drawback of this known process is that part of the ammonia leaving the spraying mechanism as a gas does not reach the bed of solid particles and is retained in the air over the bed in the drum. This ammonia is withdrawn from the drum, together with air that is also present and is recovered in a washing device by washing with an acid. It is clear that the required capacity of the washing device must be substantial as the amount of ammonia that does not react with the phosphoric acid in the drum is also substantial. In addition, when the amount of ammonia to be washed increases, there is a greater chance of reaching an NH.sub.3 /H.sub.3 PO.sub.4 ratio in the washing acid at which monoammmonium dihydrogen phosphate crystallizes and undesirable deposits are formed.